The present invention relates to a radial magnetic bearing system for use in turbomachinery, machine tools and so forth. More particularly, the present invention relates to a radial magnetic bearing system of the type having a rotor yoke rigidly secured to a rotary shaft with an electromagnet stator rigidly secured to a casing with a minute gap provided between the same and the rotor yoke, the stator being provided with an exciting coil for generating magnetomotive force, and a displacement sensor for measuring relative displacement between the rotary shaft and the casing, wherein magnetic attraction is created between the rotor and the electromagnet stator on the basis of a signal output from the displacement sensor, thereby supporting the rotary shaft in free space at or near the center of the stator.
FIG. 4 is a vertical sectional view showing a representative structure of a spindle which is supportable by a conventional 5-axes control magnetic bearing system while either FIG. 5 is a sectional view taken along the line 1--1 of FIG. 4, and FIG. 6 is a sectional view taken along the line II--II of FIG. 4.
Referring to FIG. 4, a rotary shaft 1 is driven by an electric motor which has a motor stator and a motor rotor 9 which are disposed in the central portion of a casing 7, and the rotary shaft 1 is supported by two radial magnetic bearings respectively disposed on the two sides of the motor and a thrust magnetic bearing which is adjacent to one of the radial magnetic bearings.
Each of the radial magnetic bearings comprises a radial bearing stator 3 provided with a stator coil 5, a radial bearing yoke 4 secured to the rotary shaft 1, and a radial displacement sensor 6. The thrust magnetic bearing comprises a thrust bearing stator 11 provided with a stator coil 12 and a thrust bearing yoke 10 secured to the rotary shaft 1. The reference numeral 2 denotes an emergency rolling bearing.
In this prior art, the magnetic poles of the stator 3 and the sensor elements 6a and 6b of the displacement sensor 6 are arranged as shown in FIGS. 5 and 6. Thus, the magnetic attraction force acts in orthogonal directions, that is, the directions X and Y. Displacement of the rotary shaft 1 in these two directions is detected by the displacement sensor elements 6a and 6b which are disposed in the X- and Y-directions, respectively, and the position of the rotary shaft 1 is controlled on the basis of the detected signal. The control of the position of the rotary shaft 1 in the X-direction is effected by means of magnetic attraction force which is generated between the radial bearing stator 3 and the radial bearing yoke 4 by supplying a predetermined current to either the stator coil element 5A or the stator coil element 5C on the basis of the output of the X-direction displacement sensor element 6a.
FIGS. 7 and 8 respectively show the arrangements of two examples of a control circuit in the prior art for supplying a predetermined current to the stator coils 5A, 5C, 5E, 5F.
In the control circuit shown in FIG. 7; a current from a bias power supply 23' is supplied to the stator coil elements 5E and 5F and, at the same time, a signal output from the radial displacement sensor 6 is led to a phase compensating circuit 21 and a control current generated from a power amplifier 23 is supplied to the stator coil elements 5A and 5C.
In the control circuit shown in FIG. 8, a signal output from the radial displacement sensor 6 is led to the phase compensating circuit 21 and a predetermined voltage V.sub.B is added to each of the two signals, that is, a signal output from the phase compensating circuit 21 and an inverted signal of the output of the circuit 21, and then these two signals are input to power amplifiers 23a and 23c through linear detectors 22a and 22c, respectively, thereby obtaining predetermined currents for supply to the stator coil elements 5A and 5C, respectively.
The above-described conventional radial magnetic bearing system suffers, however, from the disadvantage that four relatively costly power amplifiers are required to effect control in two directions within one plane. Further, the arrangement of the displacement sensor elements shown in FIG. 6 involves the problem that, due to thermal deformation of the casing 7, the rotary shaft 1 or the displacement sensor 6, the sensor 6 may operate erroneously as if the center of the rotary shaft 1 which is to be controlled had been displaced with respect to the center of the bearing, thus causing the rotary member to come into contact with the casing 7 or disabling the control.